Why is gas pressure specified instead of mass flow?

Question

Clarification: This question is all about the gas supply to the water heater. It has nothing to do with the water supply.

My hot water heater specifies 5.5-10.5 W.C. (water column inches) of pressure. I measured the pressure to be 7 W.C. Within spec. The problem is, this pressure reading measures the same (7wc) whether the main gas valve is (a) wide-open (b) half-open or (c) fully closed.¹

But this makes sense. Because let's assume the system pressure at the gas source is 7wc (plus a nominal pressure drop). If there is any flow at all through the pipes (and no gas is exiting through the pipe walls) then we would expect the pressure (at steady-state) at any point inside the pipe to equal the system pressure. If it were any less, then gas would flow from the source of high pressure to the point of lower pressure at the measuring point until the two pressures reach equilibrium.

So why did the engineers specify pressure? It seems like they should, at the very least, specify a mass flow rate of gas. For example, if any obstruction were inside the pipe and impeding flow or the valve were not completely open, this would result in a malfunctioning heating system. But that problem would not necessarily cause a low pressure sensor reading. But it would create an unacceptably low mass flow rate.

So what gives? Is this just an oversight? Or is there some reason I can't see why pressure and not mass flow is specified?

¹ My valve is old and it leaks. So, when it's fully shut off, its mass flow rate through the chamber is, let's say, 1% of that when it's "wide open."

Answer 1

Before I address the question: "My valve is old and it leaks. So, when it's fully shut off, its mass flow rate through the chamber is, let's say, 1% of that when it's "wide open." GET THIS FIXED IMMEDEATLY. leaky gas installations are no joke. Just because nothings happened yet does not mean that you are safe. Sorry for only noticing this now.

To your question:

The burner is, in effect, a throttle. Mass flow through the burner is then a function of the pressure at the burner.

For example, if any obstruction were inside the pipe and impeding flow or the valve were not completely open, this would result in a malfunctioning heating system. But that problem would not necessarily show up on a pressure sensor reading.

Not in the static case (valves closed), but in the static case your flow meter would sho zero too. When there's flow, a too low supply would correspond to a low pressure upstream of the burner.

agentp helpfully points out:

the input BTUH is effecively a mass flow. The heating value of natural gas is about 1000BTU/scf..so you need 360scf/

You suggest "* A good spec should specify what can be best measured.*" - correct, and that's pressure. An analog barometer is 50 € or less, an accurate gas flow meter goes for upward 1k€. Measuring gas flows is not trivial, either you need to accurately measure flow velocity and absolute pressure & temperature or you measure mass flow directly via thermal dispersion, which is more complicated than it sounds.